1. Field of the Invention
The present invention relates to nanocomposites having a matrix of silicone elastomer with multifunctional graphene sheets as filler, methods of making the same and their use.
2. Description of the Related Art
The effect of filler dispersion on the mechanical properties of the resulting composite has been studied for decades but a consensus is yet to be reached. Many have suggested that maximizing filler dispersion is crucial in achieving good mechanical properties. For example, for carbon nanotubes (CNT), Ajayan et al. suggested that load transfer can be limited when the nanotubes are slipping within the bundles.1 The bundles need to be broken into individual dispersed tube segments to obtain effective modulus increase and strengthening. Schandler et al. have proposed that infiltrating the polymer into the interstices of the nanotube bundles can create effective load transferring and therefore mechanical reinforcement.2 Similarly for inorganic fillers, Lebaron et al. have suggested that the complete dispersion of clay optimized the number of reinforcing elements for carrying an applied load and deflecting cracks, allowing for tensile property improvements.3 1 Ajayan, P. M.; Schadler, L. S.; Giannaris, C.; Rubio, A. Advanced Materials 2000, 12, (10), 750-2 Schadler, L. S.; Giannaris, S. C.; Ajayan, P. M. Applied Physics Letters 1998, 73, (26), 3842-38443 LeBaron, P. C.; Wang, Z.; Pinnavaia, T. J. Applied Clay Science 1999, 15, (1-2), 11-29
Large clusters of particles can act as flaws to initiate premature termination of stretching.4 On the other hand, it has long been suggested in the automotive tire industry that aggregated fillers are more effective than primary particles in enhancing the modulus and tensile strength of the elastomer.5 At large strains, the deformation and irreversible breakdown of aggregates absorb energy, allowing the composite to tolerate higher amounts of stress. However, a rigorous understanding of the effect of breaking up initial filler agglomerates on the mechanical properties that incorporates the two aforementioned contrasting views, is lacking. 4 Wilbrink, M. W. L.; Argon, A. S.; Cohen, R. E.; Weinberg, M. Polymer 2001, 42, (26), 10155-101805 Poovarodom, S.; Hosseinpour, D.; Berg, J. C. Industrial & Engineering Chemistry Research 2008, 47, (8), 2623-2629
In achieving the maximum effect with the minimum filler loading, it is important to understand the correlation between the spatial distribution of dispersed fillers and the macroscopic mechanical properties of the composite.6,7 Some understanding of the structure-property relationship has been developed previously by others. A larger agglomeration of silica renders a better improvement in the Young's modulus of the matrix.8 It has been shown by Akcora et al. that self-assembled nanoparticle sheet yielded a solid-like rheological behavior in polystyrene whereas well-dispersed short particle strings did not.9 However, the effect of filler assembly on the tensile properties of the composites is not yet well-understood. 6 Vaia, R. A.; Maguire, J. F. Chemistry of Materials 2007, 19, (11), 2736-27517 Balazs, A. C.; Emrick, T.; Russell, T. P. Science 2006, 314, (5802), 1107-11108 Oberdisse, J. Soft Matter 2006, 2, (1), 29-369 Akcora, P.; Liu, H.; Kumar, S. K.; Moll, J.; Li, Y.; Benicewicz, B. C.; Schadler, L. S.; Acehan, D.; Panagiotopoulos, A. Z.; Pryamitsyn, V.; Ganesan, V.; Ilaysky, J.; Thiyagarajan, P.; Colby, R. H.; Douglas, J. F. Nature Materials 2009, 8, (4), 354-U121
Another fundamental issue that has drawn much attention is the origin of the reinforcements of tensile properties in composites. Simultaneous improvements in modulus, strength and elongation at break with the incorporation of fillers have been observed in poly(methylmethacrylate),10 epoxy,11 styrene-butadiene rubber,12 polyimide,13 and silicone rubber.14,15,16,17,18 While the modulus and strength increase with the filler concentration, the elongation at break in some cases increases initially and then decreases above a critical filler concentration.11,13,16,17 10 Sui, X. M.; Wagner, H. D. Nano Letters 2009, 9, (4), 1423-142611 Tseng, C. H.; Wang, C. C.; Chen, C. Y. Chemistry of Materials 2007, 19, (2), 308-31512 Bokobza, L.; Rahmani, M.; Belin, C.; Bruneel, J. L.; El Bounia, N. E. Journal Of Polymer Science Part B—Polymer Physics 2008, 46, (18), 1939-195113 An, L.; Pan, Y. Z.; Shen, X. W.; Lu, H. B.; Yang, Y. L. Journal of Materials Chemistry 2008, 18, (41), 4928-494114 Aranguren, M. I.; Mora, E.; Macosko, C. W.; Saam, J. Rubber Chemistry And Technology 1994, 67, (5), 820-83315 Yuan, Q. W.; Mark, J. E. Macromolecular Chemistry And Physics 1999, 200, (1), 206-22016 Osman, M. A.; Atallah, A.; Muller, M.; Suter, U. W. Polymer 2001, 42, (15), 6545-655617 Bokobza, L.; Rahmani, M. Kgk-Kautschuk Gummi Kunststoffe 2009, 62, (3), 112-11718 LeBaron, P. C.; Pinnavaia, T. J. Chemistry Of Materials 2001, 13, (10), 3760-3765
The increase in modulus is attributed to load transferring to the stiffer filler material.19,20 Some understanding has been achieved in the tensile strength and elongation at break increase. Sui et al. demonstrated using transmission electron microscopy (TEM) the mechanism responsible for the significant elongation at break increase in electrospun CNT-poly(methyl methacrylate) (PMMA) fibers.10 In pure PMMA fiber, sparse and unstable necking was observed along the fiber under tension, followed by failure of the fiber. When 1.5 wt. % single wall carbon nanotubes (SWCNT) were added, multiple necking was initiated but arrested by SWCNT ropes. Further stretching led to bridging by SWCNT ropes, which caused a dilation effect in the fiber and an increase in the elongation at break. The inelastic strain and energy dissipation introduced by the necking and bridging was proposed to explain the tensile strength increase of the nanocomposite. Only one CNT concentration was used. In the same study, millimeter-sized pure and CNT filled PMMA films were studied and improvement in the elongation at break was also observed, although to a lesser extent compared to the electrospun fibers. The improvement in the films was not addressed in the study. 19 Hashin, Z.; Shtrikman, S. Journal Of The Mechanics And Physics Of Solids 1963, 11, (2), 127-14020 Nielsen, L. E. Journal Of Applied Physics 1970, 41, (11), 4626-&
Load transferring to CNT has been proposed to explain the strength and elongation at break increase in epoxy.11 When an amphiphilic block copolymer was incorporated into epoxy, elongation at break increase was observed.21 The underlying mechanisms were investigated with optical microscopy and TEM. It was found that a 15 nm size spherical block copolymer micelle could cavitate to induce matrix shear banding. It was suggested that the dilation effect and shear banding introduced by the cavitation led to the observed increase in the elongation at break. 21 Liu, J.; Sue, H. J.; Thompson, Z. J.; Bates, F. S.; Dettloff, M.; Jacob, G.; Verghese, N.; Pham, H. Macromolecules 2008, 41, (20), 7616-7624
When rod-like attapulgite was incorporated into polyimide, simultaneous improvements in modulus, strength and elongation at break were observed.13 The enhancement of the interfacial stress transfer and the resistance to crack propagation induced by attapulgite was proposed to explain the mechanical reinforcement.
Filler agglomerates acting as defects have been proposed to explain the reversal in the elongation at break.11,22 Incorporation of free volume with the filler has also been suggested to be causing the reversal effect.13 The addition of filler increased the free volume or defects in nanocomposites and the resistance to crack propagation during deformation. Below the critical concentration, the latter effect dominated and elongation at break increased. Above the threshold, the increase in the number of defects dominated and the elongation at break started to decrease. The reversal effect was also observed with the incorporation of polystyrene-modified cadmium selenide nanoparticles to polystyrene (PS).23 It was proposed that two competing effects determine the elongation at break of the composite. Nanoparticles entrapped within the mature craze during craze widening disrupt the formation of cross-tie fibrils by increasing the mobility of polymer segments at the craze-bulk interface. Less cross-tie fibrils reduced the premature rupture of the craze fibrils and increased the failure strain. On the other hand, entrapped nanoparticles also reduced the extensibility of the craze fibrils or the dilation effect of the craze. So the two competing effects led to a maximum in elongation at break of the composite as a function of nanoparticle concentrations. 22 Gorga, R. E.; Cohen, R. E. Journal of Polymer Science Part B—Polymer Physics 2004, 42, (14), 2690-270223 Lee, J. Y.; Zhang, Q. L.; Wang, J. Y.; Emrick, T.; Crosby, A. J. Macromolecules 2007, 40, (17), 6406-6412
The simultaneous improvements are not limited to polymeric matrices. The incorporation of polymeric fibers increased the strength and elongation at break of the newly engineered building material called engineered cementitious composites (ECC).24 ECCs have been designed to distribute many cracks of small width throughout the composite rather than only a few large cracks seen in traditional concrete failure. Such a distributed deformation is responsible for the observed mechanical reinforcement. Similar mechanisms have been shown to cause the elongation at break increase in biological composites such as nacre.25 24 Li, V. C.; Wang, S. X.; Wu, C. Aci Materials Journal 2001, 98, (6), 483-49225 Wang, R. Z.; Suo, Z.; Evans, A. G.; Yao, N.; Aksay, I. A. Journal Of Materials Research 2001, 16, (9), 2485-2493
Despite the aforementioned efforts, some fundamental issues governing the tensile properties improvements have not been completely understood. For example, it is not known how the filler agglomeration and filler concentration influence the interaction between fillers and tears or cracks, nor how filler length scale influences the interaction. Further, it is not known how the interaction is related to the reversal effect or how the local deformation is directly correlated with the macroscopic tensile properties in bulk composites. Lastly, it is not known how mechanical load is being transferred to the filler. These are all critical questions that need to be addressed in order to gain a complete understanding of the reinforcement.
One potential filler that has been suggested is functional graphene sheets (FGS). FGS is an atomically thin layer of graphite hundreds of nanometers in the lateral dimension and decorated with carboxyls at the edges and hydroxyls and epoxides on the planes. Our group invented a method to produce functionalized graphene sheet (FGS) on a large scale; see U.S. Patent Application Publication 2007/0092432, filed Oct. 14, 2005 and published Apr. 26, 2007 (the entire contents of which are hereby incorporated by reference; hereafter “the '432 application”). It has a wrinkled geometry with an average aspect ratio of 500 and a surface area from 300 m2/g to 2630 m2/g, typically up to 1800 m2/g.26,27 It is preferably produced through thermal exfoliation and reduction of oxidized natural graphite. The '432 application further discloses these FGS products. Stankovich et al. developed an alternative method to produce graphene.28 Graphene oxide was first obtained by oxidation of natural graphite and sonication of graphite oxide. Chemical reduction of graphene oxide yielded graphene with good electrical conductivity. In a recent study, significant increases in glass transition temperature, Young's modulus, tensile strength and electrical conductivity was observed in when 1 weight % of FGS was incorporated into poly(methyl methacrylate) and poly(acrylonitrile).29 An enhancement in the modulus and electrical conductivity as well as a reduction in the coefficient of thermal expansion and gas permeability was observed when FGS was added to poly(ethylene-2,6-naphthalate) and poly(carbonate).30,31 When reduced graphene oxide was incorporated into polystyrene, a low electrical percolation of 0.1 vol. % and good conductivities were obtained.28 26 Schniepp, H. C.; Kudin, K. N.; Li, J. L.; Prud'homme, R. K.; Car, R.; Saville, D. A.; Aksay, I. A. Acs Nano 2008, 2, (12), 2577-258427 McAllister, M. J.; Li, J. L.; Adamson, D. H.; Schniepp, H. C.; Abdala, A. A.; Liu, J.; Herrera-Alonso, M.; Milius, D. L.; Car, R.; Prud'homme, R. K.; Aksay, I. A. Chemistry Of Materials 2007, 19, (18), 4396-440428 Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Nature 2006, 442, (7100), 282-28629 Ramanathan, T.; Abdala, A. A.; Stankovich, S.; Dikin, D. A.; Herrera-Alonso, M.; Piner, R. D.; Adamson, D. H.; Schniepp, H. C.; Chen, X.; Ruoff, R. S.; Nguyen, S. T.; Aksay, I. A.; Prud'homme, R. K.; Brinson, L. C. Nature Nanotechnology 2008, 3, (6), 327-33130 Kim, H.; Macosko, C. W. Macromolecules 2008, 41, (9), 3317-332731 Kim, H.; Macosko, C. W. Polymer 2009, 50, (15), 3797-3809
U.S. patent application Ser. No. 11/543,872, filed Oct. 6, 2006 (the entire contents of which are hereby incorporated by reference), discloses the use of the FGS of the '432 application in the production of various nanocomposite rubbers.
SE has attracted both scientific and commercial interest for its thermal stability over a wide range of temperatures (−50 to over 200° C.), retention of elastomeric properties at low temperatures due to a low glass transition temperature of −125° C., its chemical and weathering resistance.32,33,34 SE is typically made by end-linking poly(dimethyl siloxane) (PDMS) and therefore its molecular weight between crosslinks is well-characterized. Due to its relatively inferior tensile strength in the unfilled state (typically less than 1 MPa, compared to more than 10 MPa of natural rubber), silica is generally used to render SE applicable in commercial applications.34,34 Other fillers including silica,14,15,35 clays,16,16,36 carbon nanotubes (CNT),17,37 graphite nanosheet,38 glass fiber,39 and in-situ precipitated alumina,40 have also been studied as alternative fillers for SE. 32 Mark, J. E. Accounts Of Chemical Research 2004, 37, (12), 946-95333 Noll, W., Chemistry and Technology of Silicones. Academic Press, Inc.: New York, 197834 Butts, M.; et. al. In Kirk-Othmer Encyclopedia of Chemical Technology—Silicones. Wiley Interscience: New York, 200435 Mark, J. E.; Jiang, C. Y.; Tang, M. Y. Macromolecules 1984, 17, (12), 2613-261636 Osman, M. A.; Atallah, A.; Kahr, G.; Suter, U. W. Journal of Applied Polymer Science 2002, 83, (10), 2175-218337 Frogley, M. D.; Ravich, D.; Wagner, H. D. Composites Science And Technology 2003, 63, (11), 1647-165438 Chen, L.; Lu, L.; Wu, D. J.; Chen, G. H. Polymer Composites 2007, 28, (4), 493-49839 Park, E. S. Journal of Applied Polymer Science 2007, 105, (2), 460-46840 Mark, J. E.; Wang, S. B. Polymer Bulletin 1988, 20, (5), 443-448